This application is based on, and claims priority to, Japanese Application No. 9-224056, filed Aug. 20, 1997, in Japan, and which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method and apparatus for reducing the amount of dispersion in an optical fiber transmission line. More specifically, the present invention relates to a method and apparatus for reducing the amount of dispersion in the transmission line by controlling the total dispersion to substantially minimize the intensity of a specific frequency component of an optical signal travelling through the transmission line.
2. Description of the Related Art
Optical transmission systems using fiber optical transmission lines are being used to transmit relatively large amounts of information. For example, optical transmission systems at 10 Gb/s are now in practical implementation in trunk-line optical communications. However, as users require larger amounts of information to be rapidly transmitted, a further increase in the capacity of optical transmission systems is required.
Time-division multiplexing (TDM) (including optical time-division multiplexing (OTDM)) and wavelength-division multiplexing (WDM) are being considered as candidates for such high capacity optical transmission systems. For example, with regard to TDM techniques, a significant amount of worldwide research is being performed on 40-Gb/s systems.
Chromatic dispersion (group-velocity dispersion (GVD)) is one of the factors limiting the transmission distance in a 40-Gb/s system. Since dispersion tolerance is inversely proportional to the square of the bit rate, the dispersion tolerance, which is about 800 ps/nm at 10 Gb/s, is reduced by a factor of 16 to about 50 ps/nm at 40 Gb/s.
For example, in measured experiments, an optical time-division multiplexed (OTDM) signal with a signal light wavelength of 1.55 xcexcm (where transmission loss in silica fiber is the lowest) was transmitted over a distance of 50 km through a single-mode fiber (SMF). The SMF had a zero dispersion wavelength of 1.3 xcexcm. This type of SMF is the type of fiber most widely installed around the world. The input signal light power was +3 dBm, and the bit rate was 40 Gb/s. Dispersion compensation was performed using a dispersion-compensating fiber (DCF). The width of the dispersion compensation value range allowed in order to hold the power penalty (degradation of optical signal reception sensitivity through transmission) to within 1 dB (dispersion compensation tolerance) was 30 ps/nm. Since the dispersion compensation value required at this time is 930 ps/nm (18.6 ps/nm/kmxc3x9750 km), it can be seen that dispersion compensation must be carried out with an accuracy of 930xc2x115 ps/nm, which is very close to 100% accurate compensation.
On the other hand, dispersion in a transmission line changes with time due to changes, for example, in temperature. For example, in the case of an SMF 50-km transmission, when the temperature changes between xe2x88x9250 to 100xc2x0 C., the amount of change of the transmission line dispersion is estimated to be as follows:
(Temperature dependence of zero dispersion wavelength of transmission line)xc3x97(Temperature change)xc3x97(Dispersion slope)xc3x97(Transmission distance)=0.03 nm/xc2x0 C.xc3x97150xc2x0 C.xc3x970.07 ps/nm2/kmxc3x9750 km=16 ps/nm.
This value can be substantial when compared with the above described dispersion compensation tolerance. Accordingly, in large-capacity transmission at 40 Gb/s and higher, transmission line dispersion must be monitored at all times to hold the total dispersion to zero. This also applies for a dispersion-shifted fiber (DSF) which has low chromatic dispersion in the 1.55 xcexcm band.
In the development of an automatic dispersion equalization system (a system for automatically controlling total dispersion to zero by feedback), the following points present problems:
(i) Realization of a variable dispersion compensator.
(ii) Method for detecting transmission line dispersion (or the amount of total dispersion after dispersion compensation).
(iii) Method for feedback control of a dispersion compensation amount.
Regarding point (i), above, a simple approach would be to use DCFs with different dispersion compensation amounts and change the amount of dispersion compensation in a discontinuous manner by switching between the DCFs using an optical switch. Methods have been proposed for continuously varying the dispersion compensation amount by applying a stress (for example, see M. M. Ohm et al., xe2x80x9cTunable grating dispersion using a piezoelectric stack,xe2x80x9d OFC ""97 Technical Digest, WJ3, pp. 155-156). In addition, methods have been proposed for providing a temperature gradient to a fiber grating (for example, see Sergio Barcelos et al., xe2x80x9cCharacteristics of chirped fiber gratings for dispersion compensation,xe2x80x9d OFC ""96 Technical Digest, WK12, pp. 161-162). Moreover, methods have been proposed for providing a phase change due to a temperature change to a planar lightwave circuit (PLC) (for example, see K. Takiguchi, et al., xe2x80x9cVariable Group-Delay Dispersion Equalizer Using LatticeForm Programmable Optical Filter on Planar Lightwave Circuit,xe2x80x9d IEEE J. Selected Topics in Quantum Electronics, 2, 1996, pp. 270-276). Another possible method would be to vary the transmission line dispersion by using a variable wavelength light source, rather than using a variable dispersion compensator. In that case, the center frequency of an optical filter must be varied simultaneously in an interlocking fashion.
Regarding point (ii), above, traditionally a pulse method or a phase method has been used that involves providing a plurality of light beams of different wavelengths and measuring group-delay differences or. phase differences between the output beams. However, using these methods during system operation requires that the system operation be interrupted during the measurement of the dispersion amount or that measuring light of a different wavelength from the signal wavelength be wavelength-division multiplexed with the signal light. In the latter case, the problem is that there arises a need to estimate the amount of dispersion at the signal light from the amount of dispersion measured with the measuring light, because the transmission line dispersion varies with wavelength. In A. Sano et al., xe2x80x9cAutomatic dispersion equalization by monitoring extracted-clock power level in a 40-Gbit/s, 200-km Transmission line,xe2x80x9d ECOC ""96, TuD, 3.5, 1996, pp. 207-210, there is disclosed a method in which the power of a clock component (B-Hz component when data signal bit rate is B b/s) is detected from a received optical signal, and the amount of dispersion compensation is controlled so as to maximize the power. This technique can be applied for the case of a return-to-zero (RZ) signal which contains a clock component, but cannot be applied for the case where the intensity of the clock component is not the greatest at zero dispersion, as in a non-return-to-zero (NRZ) signal or in an OTDM signal where a plurality of RZ signals are time-division multiplexed with their tails overlapping each other.
Regarding point (iii), above, a possible approach would be to sweep the amount of total dispersion over a wide range using a variable dispersion compensator or a variable wavelength light source while interrupting system operation, until detecting a point where the total dispersion amount becomes zero. Then, the amount of dispersion compensation can be set to that point. However, a method that can perform control at all times without interrupting system operation is preferable.
Accordingly, it is an object of the present invention to provide a method and apparatus for controlling dispersion in an optical fiber transmission line. The intensity of a specific frequency component of an optical signal transmitted through the transmission line is detected. The optical signal has an intensity v. total dispersion characteristic curve with a corresponding eye opening. The amount of total dispersion of the transmission line is controlled to substantially minimize the intensity of the specific frequency component in the eye opening. Since the eye opening is difficult to measure, the intensity v. total dispersion characteristic curve can be described as having at least two peaks. In this case, the amount of total dispersion of the transmission line can then be controlled to substantially minimize the intensity of the specific frequency component between the two highest peaks of the intensity v. total dispersion characteristic curve.
Objects of the present invention are also achieved by providing an apparatus and method for directly controlling the intensity of the specific frequency component to substantially minimize the intensity of the specific frequency component in the eye opening, or between the two highest peaks of the intensity v. total dispersion characteristic curve of the optical signal. In this case, the intensity is directly controlled, instead of controlling the intensity by controlling the amount of total dispersion.
Further, objects of the present invention are achieved by providing an apparatus and method for controlling the total dispersion of the transmission line to maintain the intensity of the specific frequency component along a point on the intensity v. total dispersion characteristic curve which is within the eye opening.
Moreover, objects of the present invention are achieved by providing an apparatus and method where a time-division multiplexed optical signal, modulated by an nxc2x7m-bit/second data signal obtained by time-division multiplexing n optical signals each amplitudemodulated by an m-bit/second data signal, is transmitted through an optical fiber transmission line. The time-division multiplexed optical signal has an intensity v. total dispersion characteristic curve with at least two peaks. Then, either (a) an nxc2x7m-hertz frequency component is detected from the time-division multiplexed optical signal after being transmitted through the optical fiber transmission line, and the amount of total dispersion of the optical fiber transmission line is controlled to substantially minimize the intensity of the detected nxc2x7m-hertz frequency component between the two highest peaks of the intensity v. total dispersion characteristic curve of the time-division multiplexed optical signal, or (b) an m-hertz frequency component is detected from the time-division multiplexed optical signal after being transmitted through the optical fiber transmission line, and then the amount of total dispersion of the optical fiber transmission line is controlled to maximize the intensity of the detected m-hertz frequency component.
In addition, objects of the present invention are achieved by providing an apparatus and method where a time-division multiplexed optical signal, modulated by an nxc2x7m-bit/second data signal obtained by time-division multiplexing n optical signals each amplitudemodulated by an m-bit/second data signal, is transmitted through an optical fiber transmission line. The time-division multiplexed optical signal has an intensity v. total dispersion characteristic curve with at least two peaks. Then, either (a) an nxc2x7m-hertz frequency component is detected from the time-division multiplexed optical signal after being transmitted through the optical fiber transmission line, and the intensity of the detected nxc2x7m-hertz frequency component is controlled to substantially minimize the intensity between the two highest peaks of the intensity v. total dispersion characteristic curve of the time-division multiplexed optical signal, or (b) an m-hertz frequency component is detected from the time-division multiplexed optical signal after being transmitted through the optical fiber transmission line, and the intensity of the detected m-hertz frequency component is controlled to maximize the intensity.
Objects of the present invention are also achieved by providing an apparatus and method for determining the total amount of dispersion in a transmission line. More specifically, the intensity of a specific frequency component of an optical signal transmitted through a transmission line is detected. Then, the amount of total dispersion of the transmission line is determined from the intensity of the detected specific frequency component.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.